#31 BB 350 Respiratory Control / Lipid Metabolism – Kevin Ahern's Biochemistry Online

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Lecture Highlights
Respiratory Control
1. Coupling refers to the fact that oxidative phosphorylation is totally dependent on electron transport and vice versa. The two processes are tied together indirectly via the proton gradient – made by electron transport and required for oxidative phosphorylation.
2. When you’re being a couch potato, your ATP concentration is HIGH, so your ADP concentration is LOW, so the CompLex V is NOT rotating (since there is no ADP), and protons are NOT moving back into the mitochondrion. The electron transport tries to continue to pump out more protons, but it CANNOT because the concentration of protons is TOO GREAT (this hill is too steep). Thus, stopping of oxidative phosphorylation STOPs electron transport in normal mitochondria. Normal mitochondria are said to be “coupled” in this way. That is to say that if one stops oxidative phosphorylation, one stops electron transport if coupling is in place.
3. Coupling also works in the other way. Consider what happens when one adds a compound, such as cyanide, that blocks flow of electrons through Complex IV. By blocking the “exit pipe” for electrons to pass to oxygen, electron transport backs up through all of the complexes and pumping of protons stops. Thus, the proton gradient is quickly dissipated when the remaining extra protons quickly pass through Complex V and are not replaced by new ones.
4. Note also that anytime electron transport is stopped, NADH and FADH2 accumulate, since they have NO PLACE to dump off electrons.
5. 2,4 dinitrophenol (2,4 DNP) pokes holes in the mitochondrial inner membrane, allowing protons to leak through it. This has the effect of UNCOUPLING electron transport and oxidative phosphorylation because protons can take a short cut around Complex V.
6. Inhibitors of electron transport stop the flow of electrons through the system. The most effective inhibitors are those, such as cyanide, that act at the last step.
7. It is important to remember that NADH does not cross the mitochondrial inner membrane. To get those electrons from glycolysis into the electron transport system, they must be “shuttled” in by another carrier. Insects use a carrier derived from DHAP (an intermediate in glycolysis) and higher animals use the aspartate-malate shuttle. The insect system is not as efficient because it generates FADH2 in the mitochondrion instead of NADH.
Lipid Metabolism
1. Fats store considerably more energy per gram than carbohydrates, due to the fact that fats are not water soluble and do not absorb water.
2. Fatty acids are released from fats or glycerosophospholipids by lipases or phospholipases, respectively.
3. Fatty acids are “activated” by being joined to Coenzyme A (CoA). To transport fatty acids across the mitochondrial membrane, however, the CoA is replaced by carnitine. After the acyl-carnitine makes it across the mitochondrial membrane, the carnitine is quickly replaced by CoA.
4. Fatty acid oxidation occurs inside the mitochondrion. The process is referred to as beta oxidation because the beta carbon of the fatty acid is the one that gets oxidized. Steps in the process include 1) dehydrogenation to form a trans fatty acyl-CoA (requires FAD, makes FADH2); 2) addition of water across the double bond (hydroxyl goes on the beta carbon); 3) oxidation of the hydroxyl to a ketone (requires NAD+, makes NADH); 4) cleavage of the 2 carbon acetyl-CoA piece at the same time as another CoA joins the end of the shortened fatty acid.
5. One cycle of beta oxidation cleaves one two carbon unit (acetyl-CoA) from the fatty acid and leaves the fatty acid shorter by two carbons. Thiolase cleaves the acetyl group from the fatty acid chain in the last step of beta oxidation. The first enzyme of fatty acid oxidation, Acyl-CoA dehydrogenase, has been linked to sudden infant death syndrome.

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